Justin, that model doesn't use an upper idle airbleed in the airhorn, it should be just above the idle down channel in the main casting.
I wouldn't worry about the POE restriction for what you are doing. For racing applications, if you get them too big, they can continue to feed the main system all the way down the track. This may produce a rich condition at full thorttle on hard runs, which can be compensated for by leaning secondary metering rods.
The larger MAB's are fine, use larger jets there, 75-77's instead of 72-74's......Cliff